The present invention relates to the field of digital radiography and more specifically to methods and apparatus for obtaining an electrical representation of a radiation image using a storage-phosphor image plate.
In the field of digital radiography a variety of methods have emerged. One such method is based on capturing the prompt-emitting light of a conventional phosphor screen with an image intensifier, a flat panel detector, or a CCD camera. Another method described in U.S. Pat. No. 3,859,527 (incorporated herein by reference for all purposes) uses a storage-phosphor plate for image detection. After being exposed to x-rays, the storage-phosphor plate is stimulated with an appropriate light source and the image recorded on the plate is read out.
Various methods for reading stored images from storage-phosphor plates have been proposed. A first method relies on a laser scanning mechanism that stimulates one pixel at a time and collects the photo-stimulated light with a photomultiplier. Unfortunately, because only one pixel is read at a time, the readout time for a typical storage-phosphor plate is unacceptably long.
In addition, the laser scanning mechanism necessary to stimulate one pixel at a time on a 14″×17″ phosphor plate is very large and complex. The stimulating laser pencil beam must remain well focused on the plate and must scan it from side to side and top to bottom with perfect accuracy. The typical size of a system for reading images from 14″×17″ storage-phosphor plates is close to the size of a household refrigerator.
Another problem relates to interplay between the dimension of the stimulating laser pencil beam on the plate (which dictates the spatial resolution of the overall reading apparatus) and the efficiency with which light released from the storage medium is collected. The larger the laser spot on the plate, the lower the resolution. As a result, typical medical storage-phosphor plate readers require the laser spot diameter to be less than 200 microns. The stimulated area of the storage-phosphor plate emits light corresponding to the intensity of the stored image at this particular location. The storage-phosphor material is therefore chosen to have the wavelength of stimulated light different from the wavelength of stimulating light so as to allow for selective collection of the stimulated light and complete rejection of the stimulating light. An optical filter is also typically used to reject the stimulating light and transmit the stimulated light. The optical filter is positioned between the plate and the photomultiplier. Ingenious light collectors have been envisioned to allow for maximum collection of the stimulated light. However, it is very difficult to achieve high collection efficiency since the stimulating light path gets in the way of the stimulated light collection device.
In addition, the stimulated light emits in all directions due to the turbid nature of the storage phosphor plate, which makes it even more difficult to collect. Conventional storage-phosphor plates are made of powder phosphor deposited on a plastic substrate. The phosphor material is granular and white, which makes the powder an almost ideal Lambertian emitter and reflector. The stimulating light from the laser beam is minimally absorbed in the plate, and mostly diffused by the phosphor granules to neighboring granules creating a spread of the laser spot on the plate. This effect results in a reduction of the spatial resolution of the plate reading system, since the region surrounding the laser spot on the plate is also stimulated.
The stimulated light created in the powder phosphor is also diffused in the plate before it reaches the surface where it can be collected. The amount of lateral diffusion of the stimulating light, and of the stimulated light, in the plate is a function of the size of the phosphor granules and the binder material. It is also determined by the thickness of the plate. Several techniques have been proposed to optimize the thickness of the plate and the size of the phosphor granules to achieve maximum performance. Thick phosphor layers are used to maximize the absorption of high energy x-rays at the expense of the spatial resolution. Thinner phosphor layers are used to maximize spatial resolution for lower energy x-ray applications. Additional optimization is achieved by placing a special layer underneath the phosphor layer, which can absorb the stimulating light and reflect the stimulated light back to the front surface. Unfortunately, because of a variety of tradeoffs, none of the previous techniques addressing the spatial resolution issues of storage-phosphor based systems has been universally effective.
For example, attempts have been made to utilize storage plates which are not made of powder phosphor, but rather of single crystal phosphor. These clear plates can potentially achieve higher spatial resolution since no light diffusion occurs in them, but they are very difficult to manufacture and extremely sensitive to scratches and mishandling.
On the other hand, with conventional powder phosphor plates, laser-based scanning systems require complex and sub-optimal tradeoffs between spatial resolution, bleaching ratio (i.e., readout efficiency), and readout speed. The maximum readout time is often dictated by the application (typically less than one minute for medical readers). The required spatial resolution limits the stimulating laser power (too strong of a laser beam creates too large of a spot) and, as a result, only a fraction of the available stimulated light is read out (i.e., partial bleaching). These tradeoffs result in a degradation of image quality (lower Detective Quantum Efficiency, i.e., DQE) since not all the information is read out of the plate. Moreover, additional steps have to be taken to erase the plate (to remove the unread information) before it is reused. Such erasures are typically achieved through intense exposure under bright fluorescent tubes.
Whereas the storage-phosphor plates themselves are ideal replacements to film-screen combinations, currently available laser-based scanning systems are far from ideal. To address at least some of the disadvantages of such systems, attempts have been made to replace the pixel-by-pixel scanning mechanism with a linescan mechanism or a two-dimensional capture device. Various linescan mechanisms have been proposed in which the laser pencil beam is replaced by a laser fan beam and the photomultiplier is replaced by a one-dimensional array of photodetectors. The idea behind such a mechanism, is to read the storage-phosphor plate one line at the time rather than one pixel at a time, thus allowing for a much faster readout time as well as a much simpler and smaller scanning mechanism, i.e., only one-axis mechanical scanning is necessary instead of two-axis scanning.
The theoretical advantages of linescanning over pixel-by-pixel scanning are clear, but the practical implementation of the stimulating fan beam and the associated collecting optics is extremely challenging. Unlike the pixel-by-pixel scanning scheme where the collecting optics are non-imaging, most linescanning schemes require the collecting optics not only to collect as much light as possible, but also image the surface of the plate onto the photodetector line array with suitable resolution. Such techniques also typically require the stimulating light to be confined to an area of the plate smaller than the area imaged onto the photodetector line array in order to guarantee that no stimulated light is lost in the process. These two requirements are very difficult to achieve with conventional techniques as evidenced by the fact that no linescanning plate reader is yet commercially available.
Numerous designs have been proposed, some relying on traditional optics (e.g., U.S. Pat. No. 5,747,825 the entire disclosure of which is incorporated herein by reference), but most assuming that traditional optics are not practical to efficiently image the surface of a plate onto a photodetector line array. In these designs, maximum collection efficiency is achieved by placing the photodetector line array in close proximity to the plate, with no conventional lens in between. Some designs suggest the use of a fiber-optic faceplate between the plate and the photodetector line array presumably to overcome certain mechanical issues related to the array bond wires.
In any case, high collection efficiency and high resolution may be achieved without a traditional lens provided that the linear array is in direct contact with the plate or that the distance between the plate and the linear array is kept to a strict minimum. This constraint creates a serious challenge as far as stimulating the area right underneath the linear array. A small gap can be placed between the plate and the linear array to let the stimulating light pass through, but since the plate has a Lambertian emission, this has a catastrophic effect on the collection efficiency and spatial resolution of the system.
Several solutions have been proposed to solve this issue. One set of solutions, proposed by Hosoi (U.S. Pat. No. 4,880,987 incorporated herein by reference), Leblans (European Patent No. 1014684 incorporated herein by reference) and Schiebel (U.S. Pat. No. 4,953,038 incorporated herein by reference), consists of utilizing a transparent phosphor plate (as opposed to a conventional turbid phosphor plate) and placing the stimulating light source on the side of the plate opposite the linear array. In this configuration, no gap is necessary between the plate and the linear array and maximum light collection efficiency and spatial resolution can be achieved. However, as discussed above, the cost of producing and handling this type of phosphor plate can be prohibitively expensive.
Another solution proposed by Kawajiri (U.S. Pat. No. 4,922,103 incorporated herein by reference) consists of placing the stimulating light source on the side of the linear array opposite the plate. This assumes that the linear array is completely transparent at the wavelength of stimulating light (so as to let the stimulating light pass through the linear array to stimulate the plate), and highly absorbing at the wavelength of the stimulated light (so as to convert the stimulated light into electrical signal). Another solution proposed by Carter (U.S. Pat. No. 4,933,558 incorporated herein by reference) consists of a row of emitting optical fibers which tips are placed at a small angle to the tips of receiving optical fibers, thus allowing the stimulating light to cross path with the stimulated light. This design has the limitations mentioned earlier relating to the gap between the plate and the receiving optical fibers.
Unfortunately, in all these proposed designs, the confinement of the stimulating light to the imaging area is a great engineering challenge. All require precise alignment and registration to ensure that no areas of the plate, other than the imaging area, are exposed. It is therefore desirable to provide techniques for reading images from storage-phosphor plates which maximize the efficiency with which image data are collected without prohibitive expense.